1. Field of The Invention
The present invention relates to a recording medium such as optical disk, optical card, and optical tape and to a process for producing the same. More particularly, the present invention relates to an improvement of a transparent plastic substrate to support the data layer. More specifically, the present invention relates to a transparent plastic substrate that can be applied to the magneto-optical recording medium and to a process for producing the same.
2. Description of Related Art
A problem encountered in the optical high-density data recording medium designed to record and retrieve submicron-sized data spots by means of a laser beam passing through a transparent substrate is a higher value of the birefringence of the transparent substrate. A recording medium having a high value of birefringence has such a low CN ratio that it is of no practical use, particularly in the case of recording medium such as magneto-optical recording medium which is designed such that the reading of data is accomplished by detecting the change of the plane of polarization which is as small as 0.1 to 0.3 degrees. The transparent substrate should preferably be produced by injection molding from polycarbonate resin because of its low cost and its resistance to change by water absorption. Polycarbonate resin, however, has a disadvantage that the birefringence is high.
The present inventors already disclosed that it is possible to greatly reduce the birefringence of the injection-molded substrate of polycarbonate by improving the molding conditions. (Japanese Patent Application No. 12565-1984 or Japanese Patent Laid-open No. 155424/1985) In their continued researches, they found that the birefringence of the plastic substrate takes place not only in the direction parallel to the flat surface of the substrate (as believed previously) but also in the direction perpendicular to the flat surface, and that the birefringence in the latter direction strongly affects the CN ratio. These findings led to the present invention.
According to the conventional method for measuring the birefringence, the linearly polarized light is sent perpendicularly to the surface of the substrate and consequently the birefringence in the direction perpendicular to the surface of the substrate is not observed. However, if the linearly polarized light is sent aslant (say 30.degree.) with respect to the surface of the substrate, the transmitted light emerges from the crossed nicols. This phenomena cannot be explained on the hypothesis that there is the birefringence only in the direction parallel to the surface of the substrate; but it can be explained if an assumption is made that there is the birefringence also in the direction perpendicular to the surface of the substrate. The detailed investigation revealed that the substrate made by polycarbonate resin has the optical anisotropism characterized by the refractive index n.sub.z in the direction perpendicular to the surface of the substrate and the refractive indexes n.sub.x and n.sub.y parallel to the surface of the substrate. In general, the absolute value of the difference between n.sub.x and n.sub.y is almost equal to 0; but the absolute values of the difference between n.sub.z and n.sub.x and between n.sub.z and n.sub.y are not equal to O. The absolute values ranges from 0.0005 to 0.0006. This means that in the case of a 1.2 mm thick optical disk there occurs a retardation of 600-780 nm in the sectional direction.
It is not known yet why the polycarbonate substrate has the same optical anisotropism as the biaxial crystal. It is apparent, however, that the optical anisotropism is attributable to the orientation of resin molecules that takes place in the mold cavity. This is explained with reference to FIG. 1 which schematically shows the behavior of molten resin in the mold cavity. The molten resin 3 receives the shear stress in the radially inward direction from the mold surfaces 1 and 2 and the radially outward force exerted by the injection pressure. Therefore, the molten resin receives three forces along the thickness direction of the mold cavity that bring about orientation in the radially inward direction, in the thickness direction, and in the radially inward direction simultaneously. The regions in which these forces are exerted are indicated by A, B, and A, respectively, in FIG. 1. It is not known which region affects which of the three refractive indexes n.sub.z, n.sub.x , and n.sub.y ; but it is believed that there are three regions in which the direction of orientation varies in the thickness direction of the substrate.
The present inventors hypothesized that it would be necessary to control the orientation in the above-mentioned region B in order to lower the high birefringence which is one of the causes that aggravate the CN ratio of a polycarbonate resin substrate. And they carried out a series of experiments based on the hypothesis to complete the present invention. The disk substrate of the present invention which has specific refractive indexes had not existed before the present disclosure, because it was impossible to determine the effect of the refractive index n.sub.z in the direction perpendicular to the surface of the substrate so long as the birefringence was measured by the conventional method, i.e., by sending the linearly polarized light perpendicularly to the surface of the substrate.